Several nucleating agents for thermoplastic polymers are known in the art. These nucleating agents generally function by forming nuclei or providing sites for the formation and/or growth of crystals in the thermoplastic polymer as it solidifies from a molten state. The nuclei or sites provided by the nucleating agent allow the crystals to form within the cooling polymer at a higher temperature and/or at a more rapid rate than the crystals will form in the virgin, non-nucleated thermoplastic polymer. These effects can then permit processing of a nucleated thermoplastic polymer composition at cycle times that are shorter than the virgin, non-nucleated thermoplastic polymer.
While polymer nucleating agents may function in a similar manner, not all nucleating agents are created equal. For example, a particular nucleating agent may be very effective at increasing the peak polymer recrystallization temperature of a thermoplastic polymer, but the rapid rate of crystallization induced by such a nucleating agent may cause inconsistent shrinkage of a molded part produced from a thermoplastic polymer composition containing the nucleating agent. Such a nucleating agent may also be ineffective in increasing the stiffness of the molded part to a desirable degree. Also, while nucleating agents for polyethylene polymers are known in the art, relatively few of these nucleating agents have been shown to improve the physical properties of the polyethylene polymer to any commercially significant degree.
Given the complicated interrelationship of these properties and the fact that many nucleating agents exhibit less-than-optimal behavior in at least one respect, a need remains for nucleating agents that are capable of producing thermoplastic polymer compositions exhibiting a more desirable combination of high peak polymer recrystallization temperature, tunable shrinkage, and high stiffness. Applicants believe that the nucleating agents and thermoplastic polymer compositions disclosed in the present application meet such a need.